1. Field of the Invention
The present invention relates to mirrors, particularly to vehicle rearview mirrors capable of automatically blocking excessive light levels, e.g., caused by headlight glare.
2. Description of Prior Art
During night driving, when a first car is followed by another car which has its bright lights on, these bright lights are reflected from the rearview mirror into the eyes of the car's driver. This can seriously impair the driver's forward vision because the light scatters within the driver's eyes, causing a "veil" or "curtain" of light over the scene. The driver therefore has reduced ability to detect objects which are dimly lit or have little contrast with the background. This situation is known as visibility glare. The driver is normally unable to detect this loss of visibility since it produces no physical sensation. At higher reflected light levels, discomfort glare occurs, resulting in an unpleasant physical sensation in the driver's eyes.
To alleviate this problem, manually actuated rearview mirrors have been developed which include "day" and "night" settings. These day-night mirrors are installed almost in all modern cars and include mirrors joined at an angle to form a prism and a mechanism for changing the angular orientation of the prism. The front mirror is half-silvered. In the day setting, the rear mirror is angularly set to the working position to provide approximately 80% reflectance. In the night setting, the front mirror is angularly set to the working position to provide only approximately 4% reflectance.
Additionally, automatic day-night rearview mirrors have been developed which automatically switch the mirror prism between full and partial reflectance states in response to sensed light levels. One such mirror is illustrated in Bauer et al. U.S. Pat. No. 4,443,057, issued Apr. 17, 1984. The Bauer mirror includes a forward light sensor for measuring light in the forward direction, a rear light sensor for measuring light in the rear direction, and a control circuit responsive to the forward and rear light sensors to control the mirror prism.
These automatic mirrors all suffer a common drawback. Specifically, the light sensors used in these mirrors have a response to the electromagnetic spectrum which is substantially different from the spectral response of the human eye. Accordingly, the sensitivities of the light sensors to visible and invisible wavelengths are different from the sensitivities of the human eye. Most notably, light sensors are extremely sensitive to infrared and longer wavelengths which cannot be seen by the human eye. Infrared wavelengths are prevalent in artificial lighting and particularly in tungsten filament bulbs. The light sensors detect relatively high light intensities when viewing headlights, taillights, streetlights, or any other source of infrared wavelengths. Consequently, the reflective element of a mirror incorporating such sensors is driven to an inappropriate reflective state. The mirror therefore is actuated when not necessary to meet the sensitivity of the human eye. Either inadequate image information is presented to the driver and/or excessive glare from the rearview mirror is directed to the driver's eye.
An attempt has been made to obviate the disadvantages of the above-mentioned light sensors by utilizing filtered light sensors of the type described in E. Gahan U.S. Pat. No. 4,799,768 issued Jan. 24, 1989. The Gahan light sensor includes a detector responsive to electromagnetic wavelengths and a filter for filtering the wavelengths received by the detector so that the spectral response of the light sensor approximates the spectral response of the human eye. Nevertheless, this rearview mirror switches into the "night" state the entire surface of the mirror. Furthermore, this mirror is based on the use of light sensors, and such sensors essentially attenuate the incident light to a certain level, or otherwise reflect the incident light in a certain direction. This causes distortion of a reflected image.
Another disadvantage, common to all two-position prism mirrors, is that such a prism is strictly a dual reflectance device which allows no option for a continuously variable reflectance or for intermediate reflectance states. Dual reflectance mirrors are highly inadequate because they distort and shift images.
A glare-free reflection mirror is shown in H. Itoh et al U.S. Pat. No. 4,721,364, issued Jan. 26, 1988. This mirror has an electro-optical element, the transparency of which is changed by applying an electric field to this element. The mirror is divided into a dazzle-free zone in the lower part of the mirror's surface and a non-dazzle-free portion at the rest of the mirror's surface. Such rough regional localization of the mirror's surface reduces the field of vision at nighttime. Furthermore, the Itoh mirror possesses the same disadvantages as the other sensor-type mirrors described above.
Recently, rearview mirrors using liquid crystal devices having light absorption properties have been designed. One such nonglaze mirror is described in K. Hara U.S. Patent 4,671,617, issued Jun. 9, 1987. Mirrors of this type incorporate a liquid crystal device. In this device, the orientations of the liquid crystal molecules are changed to absorb light when the amount of the light incident on the mirror exceeds a certain limit.
There are many other dazzle-free mirrors based on the use of liquid crystals which are described in various publications. Each such mirror is aimed at elimination of certain drawbacks of the existing liquid-crystal mirrors, such as prevention of a chemical reaction in the dichromatic dye of the liquid crystal (E. Lee at al. U.S. Pat. No. 4,848,878 issued 18, 1989), provision of control device for automatically initializing the antidazzle mirror to a selected mode of a predetermined antidazzle or dazzle state, when power is applied from a battery (H. Demura et al. U.S. Pat. No. 4,786,145 issued Nov. 22, 1988,) elimination of interference fringes which often occur under monochromatic light sources, such as sodium or mercury lamps (found over highways), or halogen lamps of automobiles (Y. Shirai U.S. Pat. No. 4,729,638), etc.
A common disadvantage of all existing liquid-crystal dazzle-free mirrors is that they cannot provide efficient attenuation of the dazzling light. This is because the light attenuation effect is distributed over the entire surface of the mirror, causing the entire mirror to become dim even through a bright light shines in only a small area of the mirror.